TF2 finished filming, and expected length.

[G1Optimal]

From here:
The first movie was 2 hours 20 minutes, so if the second is at least that length, it's no wonder the filming has taken so long to do.
That leaves about 6 months for post-production. Seems a bit tight, considering the amount of robot action we should expect in this second movie.

Maybe the CGI Animators ( or whatever they are called ) have bit more experience this time around, unlike the first movie?

and/or

They have been working on the scenes, that have already been filmed?

[MV75]

They have access to faster hardware.

A run of the mill ati 48xx card has currently ~1Tflop of processing power that can be used for physics calculations, rendering, etc. Far dwarving even an array of quad/octo core computers, and they can have up to 8 of these in a typical computer.

[SofaMan]

Also, this time they're not modelling all of the robots from scratch. They'd be able to reuse most of the models from the last film.